If you were unlucky enough to suffer a fracture, your clinician might use an implant – like a screw, pin or plate – to hold the bone in place until it heals. Most of the time, these implants are made from metals, such as stainless steel and titanium alloys.
There’s plenty to like about using metals: they’re strong and hardy, and have been shown to successfully integrate with the surrounding tissue. But they also have disadvantages. For one, there’s a possibility that the implant will eventually need to be removed. This calls for another surgical procedure, introducing a new risk of complications and injury. Metal implants can also contribute to stress shielding, where the implant bears load that the bone would normally manage. This can lead to bone loss.
Instead, you could use resorbable polymer implants. These degrade as the tissue heals, so they don’t need to be taken out. While modern resorbable polymer fixation devices have seen development and use since the 1960s, the concept of resorbable medical materials, such as absorbable sutures, dates back much further. Efforts to create diverse resorbable implants have been ongoing for decades, with varying degrees of success. Yet from screws to stents to tissue regeneration, the potential of these implants is immense.
Popular polymers
The main class of polymers used in resorbable implants belongs to a family called poly α-hydroxy esters. And for good reason: as well as being biodegradable and biocompatible, they’re widely accessible and have a history of safe use.
Polyglycolide (PGA) and polylactide (PLA) are two of the most prominent examples. PGA is a high-strength polymer that’s typically completely resorbed within a few months, whereas PLA degrades comparatively slowly. They can also be combined to create the copolymer PLGA, says Paul Hatton, professor of biomaterials science at the University of Sheffield’s School of Clinical Dentistry. “By making PLGA, you can kind of tune the resorption rate.”
These polymers are also known for their mechanical properties, adds Reece Oosterbeek, associate professor of engineering science at the University of Oxford. “PLA and PGA, and copolymers of those, relative to other polymers, are reasonably strong. So, they’re attractive for the types of applications we’re interested in.” For instance, in orthopaedic devices where an implant needs to support the load while the tissue around it is healing.
However, they have a notable drawback: as they absorb water, they erode in bulk. That is, rather than degrading layer by layer – called surfaceeroding behaviour – the entire implant is affected by degradation throughout its bulk, leading to a progressive loss of mechanical properties over time. While this behaviour can be predicted to an extent, it makes it more difficult to forecast and control the implant’s mechanical properties over time. “What we would really like is surface eroding behaviour,” Oosterbeek says, which is much more predictable.
To some extent, the degradation rate of a polymer can be managed. Reducing its crystallinity can accelerate breakdown, while other strategies – such as limiting water absorption, increasing hydrophobicity, or altering the material’s underlying chemical structure – can influence how quickly chemical bonds are broken, thereby slowing the degradation process.
Plus, when the implants do degrade, they can have undesirable knock-on effects. Oosterbeek gives an example: PLA breaks down into lactic acid, which creates an acidic environment around the tissue. “If that builds up too much, that can trigger an unfavourable response in the tissue around it.”
All these factors must be considered when designing an implant. As well as ensuring it’s the right shape and stiffness, and will degrade in an acceptable way, you need to be sure it won’t cause harm, says Hatton. “You’ve got to get the right polymer in the right form, in the right situation.”
Fixing in place
When it comes to load-bearing implants like orthopaedic fixation devices, the strength of the polymer is key. “Polymers like PLA and PGA tend to be favoured, where it’s really about being able to support that mechanical load,” says Oosterbeek.
For instance, noted in a 2018 paper in the journal HAND, resorbable implants can have comparable biomechanical properties to metal ones when it comes to fixation of the metacarpal shaft. Resorbable implants might also reduce stress shielding, because the implant transfers weight slowly to the healing bone over time. Yet, notes Hatton, who works across orthopaedics and dentistry, these polymers aren’t as strong as titanium, so you need to use more material. They are also less reliable and carry the risk of inflammation, he adds.
“PLA and PGA, and Copolymers of those, relative to other polymers, are reasonably strong. So, they’re attractive for the types of applications we’re interested in.”
Associate Professor Reece Oosterbeek
PLLA, a version of PLA that’s hydrophobic and crystalline, has been used in interference screws and plates to fixate tissue and bone. In fact, when it comes to fractures, PLLA is a popular material choice due to how slowly it resorbs, notes a 2012 paper in the Journal of Healthcare Engineering. However, the authors note that while suitable for many applications, PLLA may not be strong enough for sole support in all larger, high-load-bearing fractures, such as certain types of femoral fractures, where metallic implants often remain the preferred option. With PGA, because it degrades – and therefore, loses its mechanical strength – faster, it’s been used in rods and screws for fractures of cancellous bone, the less dense, spongy inner layer of bone.
You can also combine materials to give an implant its desired properties, Oosterbeek explains. He gives an example: PLA can be used alongside hydroxyapatite. “The hydroxyapatite as a ceramic particle will have some mechanical role, it’ll make the material stiffer. But it’s also there as an osteo-conductive material to trigger that behaviour in the physiological environment. So, it has both a mechanical and a biological role.”
Repairing tissue
Resorbable polymers can be used to create scaffolds for tissue regeneration, too. Here, the implant acts as a sort of framework for cells to grow on. These scaffolds “are often porous, because you want some kind of diffusion of oxygen and biological molecules”, says Hatton. “And you want those cells to start forming the tissue and taking over the functions you want.”
In dentistry, while the mouth’s sensitivity necessitates careful consideration of inflammation, PLA and PGA are nonetheless utilised in tissue scaffolds and other applications. The risk of inflammation is managed through optimised material design and appropriate clinical application, rather than by broadly avoiding these polymers. Work is under way to develop resorbable membranes, but the path to market is challenging. “The evidence is that it really would work. The problem is that to get a tissue engineering product to market is ten times the cost of just the membrane on its own,” Hatton explains.
A limited number of scaffold products are currently available. There are also a handful of bone scaffolds and resorbable stents, which can support the blood vessel as it heals while encouraging regrowth of injured tissue.
Because stents need to be quite strong, the material requirements are similar to orthopaedics, says Oosterbeek. Yet polymers like PLA don’t tend to be strong or ductile enough to use in cardiac stents, he adds. So, the struts of the stents tend to be bulkier, “which leads to an increased risk of clotting down the line”. It’s a challenge many are working on: other materials such as polycaprolactone (PCL) are currently being investigated.
While some have used PLA to create scaffolds, it’s easier to create porous architecture using PCL, Oosterbeek explains: “For a pure scaffold application, it’s generally less about the mechanics and more about the biocompatibility. You would be looking at: do cells grow on this material? While PCL might not be what we call osteo-inductive – it might not induce cells to produce bone – it will be biocompatible. It will allow growth on it, and then people will try to incorporate other elements into a composite to induce that cellular behaviour, by using things like glasses.”
It’s something Oosterbeek is currently exploring – creating implants that combine resorbable polymers with bioglass. There are bioglasses that bond very well to bone, and that can help induce growth of both bone and blood vessels, he explains. “All of this is very beneficial for an implant.”
Resorbable polymer vs resorbable membrane
While both are designed to disappear inside the body, resorbable polymers and resorbable membranes serve distinct roles in medical implants.
Resorbable polymers – like PLA, PGA and PLGA – are the foundational materials used to build scaffolds and implants. These tend to be denser, offering structural support where needed, and are engineered to degrade safely over time, aligning with the mechanical properties of native tissue.
In contrast, resorbable membranes are thin, fl exible sheets made from these polymers, designed primarily as barriers; for example, to guide tissue regeneration or separate healing zones. Their lower density makes them ideal for non-loadbearing applications.
Both play a crucial role in advancing next-generation, surgery-free implant solutions.
Dental membranes
The purpose of a dental membrane is to prevent gum from growing into the bone cavity. Dental membrane is placed over the bone but under the gum. Resorbable membranes, also known as dissolvable membranes are made from materials such as collagen (porcine or bovine derived), laminar bone, connective tissue transplants as well as resorbable synthetic membranes. They have excellent protective capabilities and great handling properties, and will dissolves on its own.
Source: United Kingdom Dental Membranes Market Outlook to 2033 – Non-Resorbable Membranes and Resorbable Membranes, GlobalData
